Adding a purple is pretty much only done when using autofire, multiple weapons, or upgrading past all reds.

Anything external to the acting character and the target of the action is blue or black. Cover adds a black. concealment adds a black or two. Distractions add a black or two. Terrain and weather penalties add blacks.

Help adds a blue. Suggested not to allow more than 1 helper in the rules; I suggest that it be limited to skilled helpers most of the time, and be limited by the leadership skill of the person receiving the help as to number of helpers.

Note that the two weapon rule can be mangled slightly into a two action rule...
take the higher difficulty, add 1, and if success, one of the swo succeeds, and two advantage are needed to do the second, both get full benefit of all rolled successes.

Note also: Gunnery difficulties are different between the two games (Genesys vs SW).